master servant distance

Servant and Master will act completely apart.

To begin with, there is no range limit for the contract between the two. Whatever the distance, a Command Spell can control the Servant, and prana can still be supplied as well unless the Master faints. But Master and Servant actually staying together is solely a mutual understanding. Prudence dictates that battle decisions aren't just left to the Servant. The Master needs to remain on the battle scene to control the Servant.

If Kiritsugu moves independently without knowing about Saber's actions, of course that doesn't mean he trusts her completely. Kiritsugu entrusts to Irisviel the duty of being his agent to oversee Saber’s actions.

That's not something so reckless at all. For example, if Kiritsugu's Servant were to act rebellious, there is no fear she would want to kill Irisviel as long as she wants the Grail. Without Irisviel, then even if Saber defeats all the other Servants, she won't be able to reach the Grail. The "vessel of the Grail" that Irisviel carries is absolutely necessary for the Grail of Fuyuki to materialize. Therefore, Saber will need to defend Irisviel from the other Masters.

- Fate/Zero: Act 2, Part 2